1. Field of the Invention
The present invention relates to a needle threading device used in a sewing machine, for automatically passing an upper thread through an eye of a sewing needle when a lever provided to the sewing machine is pressed down.
2. Description of the Related Art
A sewing machine, such as a household electrically controlled sewing machine and an industrial sewing machine, includes a motor for driving a needle bar to move in the vertical direction via a principle shaft. The needle bar detachably supports a sewing needle at its lower end. Stitches are formed by cooperative operation between the vertically moving sewing needle and a loop taker that is driven in synchronization with the vertical movement of the sewing needle.
A variety of different types of needle threading devices for automatically threading the sewing needle have been provided for use in the sewing machine.
For example, Japanese Patent-Application Publication (Kokai) No. HEI-3-133485 discloses one type of needle threading unit having a needle threading hook and a thread catching mechanism. The thread catching mechanism includes an upper plate and a lower plate for pressingly holding the upper thread therebetween. The lower plate is formed with a groove on its upper surface. With this configuration, the hook is pivoted at a position level with the needle eye of the sewing needle, and passes through the needle eye. Because the upper thread is provided in front of the needle eye at this time, the hook catches on the upper thread. When the hook pulls out the upper thread through the needle eye, the upper thread gets caught in the groove of the lower plate. As a result, the pressing force on the upper thread between the upper and lower plates is released. Therefore, the upper thread is freely supplied from the free end portion of the upper thread by the amount required for the hook to draw out the upper thread through the needle. In this way, the sewing needle can be threaded.
However, when the user wishes to use an upper thread with a thickness greater than the width of the groove, the upper thread will not fit into the groove. As a result, pressing force on the upper thread cannot be released, so the sewing needle cannot be threaded. In order to overcome this problem, it is conceivable to form the groove with a greater width. However, in this case, when a thin upper thread is used, the upper thread can undesirably enter the groove easily when the hook moves to the position level with the needle eye. As a result, the upper thread cannot be provided in front of the needle eye when the hook passes through the needle eye. In this case also, the sewing needle cannot be threaded.
Japanese Patent Publication (Kokoku) No. HEI-7-24715 discloses a sewing machine capable of automatically threading a sewing needle by using a linking mechanism that links movement of a hook and a tension disk. Specifically, when a slider is lowered, a hook is pivoted at a position level with the needle eye of the sewing needle, and passes through the needle eye so as to hook on the upper thread. Simultaneously, a member of the linking mechanism pivots in linking association with lowering movement of the slider. The pivoting movement of the member releases pressing force of the tension disk on the upper thread. As a result, the upper thread is supplied from a thread spool via the tension disk by the amount required for the hook to pull out of the needle eye.
However, this configuration requires the linking mechanism including a plurality of members, and increases the number of required components. Also, because pressure of the tension plate is released by the member, the position of the thread tension mechanism is restricted to a location accessible by the member.